Recently, from the viewpoint of increase in environmental consciousness, liquid fuels in which the contents of environmental load substances such as sulfur and aromatic hydrocarbons are small have been demanded. From such a viewpoint, as a technique which can produce a base stock for fuel oil that substantially contains neither sulfur nor aromatic hydrocarbons and is rich in aliphatic hydrocarbons, particularly, a base stock for kerosene and gas oil, a technique that has been attracting attention is one in which synthesis gas (mixed gas containing carbon monoxide gas and hydrogen gas as main components) is produced from a hydrocarbon source such as natural gas by a reforming reaction, hydrocarbons are synthesized from this synthesis gas by a Fischer-Tropsch synthesis reaction (hereinafter, also referred to as the “FT synthesis reaction”), and the hydrocarbons are further refined by hydroprocessing and fractionating to thereby obtain a base stock for fuel oil (see Patent Literature 1, for example). This technique is called GTL (Gas To Liquids) process.
A synthetic oil obtained from synthesis gas by the FT synthesis reaction (hereinafter, also referred to as the “FT synthetic oil”) is a mixture containing aliphatic hydrocarbons with a wide carbon number distribution as a main component. A naphtha fraction, a middle distillate fraction, and a wax fraction can be obtained by fractionating this FT synthetic oil according to boiling points. Among the respective fractions, the middle distillate is the most useful fraction corresponding to a base stock for kerosene and gas oil and is desired to be obtained with a high yield.
Meanwhile, in the FT synthesis reaction, in addition to saturated aliphatic hydrocarbons that are main products, olefins and oxygen-containing compounds such as alcohols containing an oxygen atom derived from carbon monoxide are produced as by-products, and these by-products (impurities) are contained in large amounts in the naphtha fraction or the middle distillate obtained by fractionating the FT synthetic oil. In the case of using hydrocarbons containing these impurities as fuels, there is the possibility that constituent materials of engines may suffer damage, and therefore, it is necessary to remove these impurities. This removal of the impurities can be performed by hydrotreating the hydrocarbon oils such as the naphtha fraction and the middle distillate containing the impurities in an upgrading step of refining the FT synthetic oil by the GTL process.
In addition, since the hydrocarbons produced by the FT synthesis reaction are basically straight-chain aliphatic hydrocarbons and the straight-chain aliphatic hydrocarbons are highly crystalline, a fuel oil containing these in large amounts loses cold flow property (fluidity in a low temperature). Therefore, for the middle distillate that serves as a base stock for kerosene and gas oil, it is necessary to convert the straight-chain aliphatic hydrocarbons to branched-chain hydrocarbons by hydro-isomerization, thereby modifying cold flow property. This hydro-isomerization is generally performed simultaneously with the hydrotreating.
In the hydrotreating step of performing hydrotreating accompanied by the hydro-isomerization of the middle distillate, a hydrotreating catalyst is used in which an active metal that is selected from noble metals of Group 8 to Group 10 in the periodic table and has a hydrogenation activity is supported by a catalyst support having solid acidity such as zeolite and/or an amorphous composite metal oxide (see Patent Literatures 2 and 3, for example).
The hydrotreating catalyst used in the hydrotreating step for the middle distillate has, in addition to an activity for the above-described hydrotreating and hydro-isomerization, also an inevitable activity for a cracking reaction (hydrocracking reaction), that is, a reaction that cleaves carbon-carbon bonds in hydrocarbons to reduce the molecular weights. Therefore, when the middle distillate is subjected to the hydrotreating step, a light fraction having a boiling point lower than the boiling point range of the middle distillate is partially produced. Since the production of this light fraction leads to reduction in the yield of the middle distillate, it is preferable to suppress it.